Certain fermentation processes cannot be carried out or, are carried out inefficiently in conventional stirred-tank type fermentors. One class of such fermentation processes is in the continuous production of secondary metabolites, for example, extracellular enzymes, wherein product formation lags behind microbial growth; in this example, a fermentor with plug-flow of fluid through the apparatus would theoretically give a higher productivity than one with mixed-flow.
Another class of such fermentation processes is one in which solid material is continually deposited on the fermentor vessel walls such as from microbial growth, especially of filamentous organisms, polymeric metabolites, or media particulates such as found in the microbial degradation of solid substrates.
Yet another class of such fermentation processes is one in which only mild agitation conditions are required such as for large filamentous growths or media film-surface growths.
In a plug flow of material through a pipe, vessel or the like, individual elements of material do not mix longitudinally in the direction of flow of the material and hence may be considered as isolated from each other. This is in complete contrast to the conventional mixed-flow pattern of a stirred-tank fermentor in which the contents of the process materials are completely mixed throughout the vessel.